Push rod pinch on restricted intake manifold.

[RobZ28]

I have a very long and narrow intake with many bends, total length is about 21" with the cylinder head. Were talking a labyrinth of turns, bends and doglegs. The average CSA is right about 2.08 or so and the RPM is calling for a 1.80-1.90 PRP. Intake flow will be right around 260 if I can reach 300fps. My question is, if the intake system FPS is slow enough as it is, what can you do with the PRP MCSA? Narrowing this may speed up the pinch but when the intake is not much bigger anyway, would opening up the PRP create more intake airspeed and actually help average out the system by making the valve more of the choke point? I know we have to have the PRP move air around a turn, and that it has to be there, but what if that air is already slower and can make the turn easier than a well flowing induction system? Usually the answer to opening up the pinch would be NO. Its going to kill your lower end tq and so forth but this system is pretty slow and narrow to begin with. The valves in the head are currently 1.94" but they can be opened up further if need be. Im looking at trying to increase the average airspeed in an entire system that is tapped out for cross sectional area. Increasing the intake CSA would be beneficial for pulling/pushing that air through the Labyrinth of curved runners and sharp turns.

The theories that are being suggested is the differential between the slow and fast localized areas are what creates a great cylinder head and you want to reach your port limiting velocity near your shift point.

Any ideas or input welcome.

[mag2555]

Some things to keep in mind.
Expansion and contractions in the intake tract will cause the power band to narrow up even if the tract is mostly dead straight .
Now add some bends to that and things get even worse because high speed air no less air ladened with fuel will not make it around a bend of more then 15 degrees without loosing some numbers.
In bend areas as such the outside wall should be kicked over and or the roof raised.

So every 15 degree bend in the system will act as another pinch point of steps are not taken to maintain the cfm
Numbers previously had before the bend.

[RobZ28]

Some things to keep in mind.
Expansion and contractions in the intake tract will cause the power band to narrow up even if the tract is mostly dead straight .
Now add some bends to that and things get even worse because high speed air no less air ladened with fuel will not make it around a bend of more then 15 degrees without loosing some numbers.
In bend areas as such the outside wall should be kicked over and or the roof raised.

So every 15 degree bend in the system will act as another pinch point of steps are not taken to maintain the cfm
Numbers previously had before the bend.

Thank you for the reply! The engine is EFI so luckily we can take wet flow out of the equation. At this point, it looks like a bigger crank is going to be the only way to accomplish my goals. The whole idea here is open the PRP a little more than usual to help the signal through the intake manifold, which is lacking in velocity due to shape and length.

[Tom68]

? I know we have to have the PRP move air around a turn, and that it has to be there, but what if that air is already slower and can make the turn easier than a well flowing induction system? Usually the answer to opening up the pinch would be NO. Its going to kill your lower end tq and so forth

Hurt your mid range torque, low end torque isn't so fussed about air speed (unless your talking about a Diesel that doesn't ever have to rev high).
All depending what you're calling low end of course.

[RobZ28]

So just to be clear, my CSA is 2.08" - Looking at an AFR Vortec head, the PRP is 2.07" while we are looking at using a 1.90".

Air accelerates through the pushrod pinch correct? So if I have an intake port already sized for 300-311fps, why would I want the air faster than that with a smaller pinch? Especially when it has to go through a TPI induction system. Can anybody help me out? Are we supposed to treat the entire intake port as a singular port or not? I could see on a bigger port where your airpeeds in the intake may be lower and you want good street drivability so you use a smaller pinch but this seems like it could be a different animal.

FWIW, we are looking into different PRP shapes because the intake to head angle is so bad the flow is all on the roof of the intake port.

Thanks.

[RobZ28]

? I know we have to have the PRP move air around a turn, and that it has to be there, but what if that air is already slower and can make the turn easier than a well flowing induction system? Usually the answer to opening up the pinch would be NO. Its going to kill your lower end tq and so forth

Hurt your mid range torque, low end torque isn't so fussed about air speed (unless your talking about a Diesel that doesn't ever have to rev high).
All depending what you're calling low end of course.

This is a TPI engine. It has LOADS of low and midrange TQ. Also guys have ran as big as a 2.07" pinch with no problems.

I honestly think the pinch should be a little bigger to help the top end breath. It might sacrifice some low and midrange, but theres no way itll kill it with 20" of intake runner designed for the 2nd harmonic, which coincidentally is the max tq harmonic.

[Stan Weiss]

260 CFM air flow

1.8 sqin CSA - Average Air Speed 346.7 fps

1.9 sqin CSA - Average Air Speed 328.4 fps

2.0 sqin CSA - Average Air Speed 312.0 fps

2.08 sqin CSA - Average Air Speed 300.0 fps

Stan

[skinny z]

260 CFM air flow

1.8 sqin CSA - Average Air Speed 346.7 fps

1.9 sqin CSA - Average Air Speed 328.4 fps

2.0 sqin CSA - Average Air Speed 312.0 fps

2.08 sqin CSA - Average Air Speed 300.0 fps

Stan

When discussing CSA (in this case) is that total intake tract average? Or just the PRP? Is that air speed average through a given area at that particular point or overall?
I'm somewhat familiar with "cylinder head math" but it's the use of the word "average" that has me asking.